1. Field of the Invention
The present invention relates to a wire-cut electric discharge machine having a wire tension control function.
2. Description of the Related Art
In a wire-cut electric discharge machine, a tension is applied to a wire electrode as the electrode is run, a voltage is applied between a workpiece to be machined and the electrode to cause electric discharge, and the workpiece is machined by the electric discharge. In order to apply the tension to the wire electrode, a take-up roller is driven by a motor to take up the electrode at a fixed speed. Further, the tension is applied to the electrode by using a brake roller that is provided with resistance applying means, such as a powder brake, for controlling torque.
In a known wire electrode tension control, a first motor for feeding a wire electrode to a workpiece and a second motor for delivering the wire electrode from the workpiece are located on the wire feed side and takeout side, respectively, of the workpiece. In this tension control, the tension on the wire electrode is detected by means of a tension detector, and the speed of the motor is controlled so that the difference between the detected tension and a preset tension is zero (see Japanese Patent Application Laid-Open No. 7-328849).
In another known wire electrode tension control, capstans that run a wire electrode held by pinch rollers are disposed individually on the upstream side and downstream side with respect to the feed of the electrode to a workpiece, and a tension on the wire electrode is controlled in a manner such that the upstream capstan is rotated at a fixed speed and the downstream capstan is given a fixed torque that acts in the same direction as the direction of the rotation. In still another known wire electrode tension control, a tension on a wire electrode is controlled in a manner such that the rotational speed of a downstream capstan is fixed and an upstream capstan is given a fixed torque that acts reversely to the rotating direction. In a further known wire electrode tension control, a tension is applied to a wire electrode in a manner such that the travel of the electrode is braked by making the peripheral speed of an upstream capstan lower than that of a downstream capstan (see Japanese Patent Application Laid-Open No. 2001-328029).
In wire-cut electric discharge machining, a wire electrode delivered from a wire bobbin is arranged along a traveling path, including guide rollers, a feed roller, a machining start hole or groove in a workpiece, delivery rollers, etc. This arrangement of the wire electrode along the traveling path is called wire connection. The electric discharge machining is performed with the feed roller and the delivery rollers driven to run the wire electrode as a voltage is applied between the electrode and the workpiece.
When the travel of the wire electrode is started in order to connect the electrode, thereby starting the electric discharge machining, the guide rollers and the like are stationary. At the start of travel of the wire electrode, the static friction force and inertial force of the guide rollers act as a tension on the electrode, and this tension is unstable. In many cases, therefore, the control is so unstable that the wire electrode inevitably breaks despite the application of the prior art techniques in which the tension control is performed based on speed control of the motor that drives the travel of the electrode.
The wire electrode sometimes must be manually delivered when the wire bobbin is to be replaced with a new one or if the electrode is broken during machining. Conventionally, in manually delivering the wire electrode to be connected, a feed motor that feeds the electrode toward the workpiece is never driven, and its speed control is disabled. In order to reduce costs and simplify the structure, moreover, a speed reduction mechanism is generally used for the motor that drives the travel of the wire electrode. Thus, if the wire electrode is to be manually taken out with the motor drive stopped, the operation entails use of an additional force FR (=force (F) for no-load slip torque of motor×deceleration ratio (R)). Thus, the manual delivery requires a considerable force.
In general, on the other hand, the diameter of a wire electrode ranges from 0.02 mm to 0.4 mm. If the diameter is not larger than 0.15 mm, the wire electrode may possibly be broken by an excessive tension when it is manually delivered.